Khalid Mohammed, Associated Press
An Iraqi soldier searches a motorist at a vehicle checkpoint in Baghdad. Insurgents are adopting new, deadlier tactics, an Iraqi military spokesman says.

BAGHDAD, Iraq — The U.S. military warned Thursday that insurgents are adopting new tactics in a campaign to spread panic after troops uncovered a car bomb factory with propane tanks and chlorine cylinders — possible ingredients for more chemical attacks — following three explosions involving chlorine.

Those blasts and a recent spate of attacks against helicopters have raised fears that insurgents are trying to develop new ways to confront U.S. and Iraqi forces. Any increase in chemical bombings could complicate the Baghdad security crackdown, now in its second week.

Lt. Gen. Ray Odierno, the No. 2 American commander in Iraq, said he did not think the attacks signaled a more capable insurgency. Instead, he said, they were merely an attempt to provoke fear.

"What they're trying to do is ... adapt in such ways where they can continue to create instability," Odierno said.

The general also said at least two suspects have been arrested in the downing of eight helicopters since Jan. 20, but he gave no further details.

The raid on the car bomb factory occurred late Tuesday in the volatile western province of Anbar, U.S. authorities said. U.S. troops discovered a pickup truck and three other vehicles that were being prepared as car bombs, as well as detonation material in five buildings.

"We also found ingredients to be used to devise or enhance explosives, such as fertilizer and chlorine cylinders," Odierno told Pentagon reporters by video-link.

Insurgents have detonated three trucks carrying chlorine canisters since late January. The most recent attack occurred Wednesday in Baghdad, killing five people and sending more than 55 to hospitals.

On Tuesday, a bomb planted on a chlorine tanker left more than 150 villagers stricken north of the capital. More than 60 were still under medical care Wednesday.

A suicide bomber driving a dump truck filled with explosives and a chlorine tank also struck a quick reaction force and Iraqi police in the Sunni city of Ramadi on Jan. 28, killing 16 people.

Jeremy Binnie, an analyst with Jane's Terrorism and Insurgency Center, noted that it is unclear how many in the attacks died from the explosions and how many were victims of the chlorine itself.

U.S. and Iraqi officials pledged to adapt to fight the evolving insurgent tactics.

"What is obvious to us is that the terrorists are adopting new tactics to cause panic and as many casualties as they can among civilians," Iraqi military spokesman, Brig. Gen. Qassim Moussawi, told reporters. "But our plans also are always changeable and flexible to face the enemies' new tactics."

Although relatively few people have been killed or seriously injured in the chlorine blasts, such attacks are unnerving and can cause panic among a people suffering severe psychological strains after nearly four years of war.

With low levels of exposure, chlorine, which was used as a weapon in World War I, can cause breathing problems and irritate the skin. At high levels, it is fatal.

Experts say chlorine is used as a disinfectant and is widely available in Iraq, which the U.N. long suspected of trying to build a chemical weapons arsenal.

But former U.N. weapons inspector David Kay said U.N. experts paid little attention to "garden variety" industrial-grade chemicals such as fertilizers and chlorine unless they were found in large quantities or near weapons plants. Major cities such as Baghdad stocked chlorine for water treatment.

The discovery of the car bomb factory Tuesday took place in the town of Karmah, 50 miles west of the capital.

Elsewhere in Anbar province, Sunni insurgents armed with rocket-propelled grenades attacked U.S. troops in the volatile city of Ramadi, setting off a six-hour firefight that killed at least 12 people, the U.S. military said Thursday. Iraqi authorities said the dead included women and children.

The battle broke out Wednesday evening when insurgents opened fire on a U.S. patrol from nearby buildings. The Americans responded with "precision guided munitions" that damaged several buildings and ended the fight, Marine spokesman 1st Lt. Shawn Mercer said.

There were no U.S. casualties, but 12 insurgents were killed and three were wounded, Mercer said. He said no civilian casualties were reported.

However, Dr. Hafidh Ibrahim of the Ramadi Hospital said the bodies of 26 people, including four women and children, were pulled from the rubble of three houses damaged in the fighting.

Photographs made available to The Associated Press showed the bodies of two small boys wrapped in one blanket. Other photos showed four or five bodies covered by blankets, and several men clearing rubble.

Firefights are not unusual in Ramadi, the Anbar provincial capital 75 miles west of Baghdad. The clashes underscore the challenges posed by Sunni insurgents in the area even as the U.S. seeks to quell Sunni-Shiite violence in the capital.

President Bush is sending 21,500 more soldiers to Iraq — 4,000 of them to Anbar and the rest to Baghdad for the security plan.

One U.S. soldier was killed and three others were wounded in a roadside bombing Thursday near the Shiite city of Diwaniyah, the U.S. command said.

Nationwide, at least 19 people were killed or found dead Thursday, including 14 bullet-riddled bodies showing signs of torture that were found in Baghdad and two in the southeastern city of Kut. Three others were shot to death in the northern city of Mosul.

Also Thursday, an Iraqi official said four Iraqi soldiers were accused of raping a 50-year-old Sunni woman and the attempted rape of her two daughters — the second allegation of sexual assault leveled against Iraqi forces this week.

Brig. Gen. Nijm Abdullah said the alleged attack took place about 10 days ago in the northern city of Tal Afar during a search for weapons and insurgents.

A lieutenant and three enlisted men denied the charge but later confessed after they were confronted by the woman, a Turkoman, Abdullah said. He said a fifth soldier suspected something was wrong, burst into the house and forced the others at gunpoint to stop the assault.

A second rape allegation within a single week is likely to undermine further the reputation of Iraq's security services, which the U.S. hopes can take over from coalition troops so the Americans and their allies can go home.

The al-Qaida in Iraq leader, Abu Hamza al-Muhajir, also known as Abu Ayyub al-Masri, purportedly called on his followers to step up attacks on Iraqi security forces to avenge the alleged rapes.

He also claimed in an audio tape that 300 followers have volunteered for suicide missions within hours of hearing news of the alleged rape in Baghdad, which the woman said took place in a police garrison.

Of the volunteers, 50 are members of her tribe and 20 expressed a willingness to marry her, he said.

"Go ahead with Allah's blessing and engulf their checkpoints in fire, destroy their homes, and spill their blood to flow as streams," the terror leader said.

The authenticity of the tape could not be immediately verified, but the voice sounded like al-Masri's and it appeared on Web sites commonly used by the militant groups.